Laser interference lithography involves using a laser beam to produce fringe patterns on a layer of photoresist. A basic form of laser interference lithography uses two overlapping and coherent laser beams. The two overlapping beams may be used to produce straight line interference fringe patterns in two orthogonal directions on a layer of photoresist. The exposure of the photoresist to the beams may be conducted in two steps. In step one, the photoresist is oriented so that exposure to the beams creates a fringe pattern in a certain direction, for example the x-axis. In step two, the photoresist layer may be rotated approximately 90 degrees so that the resulting fringe pattern is in a direction orthogonal to that created in step one, for example the y-axis. The resulting overall fringe pattern consists of two sets of straight line interferences perpendicular to one another.
Laser interference lithography may be employed on either positive or negative tone photoresist. In the case of positive tone resist, the exposed regions of the photoresist are developed away and the unexposed regions remain as resist dots or bumps. In the case of negative tone resist, the exposed regions remain while the unexposed regions develop away giving rise to holes in the photoresist. Thus, either resist dots or holes can be generated using this technique, in addition to lines and spaces. Typically, due to the surface tension of the resist material, what would otherwise be square features resulting from the laser interference develop rounded comers. Other patterns may also be generated using this method, such as elongated dots, holes or mesa structures.
The feature size of the fringe patterns is determined by various exposure and development parameters. However, feature size is also influenced by the feature-to-feature spacing, which can be determined from the following equation: EQU 2dsin.theta.=.lambda.
where d is fringe spacing, .lambda. is the wavelength of laser light, and .theta. is the half angle between the two beams. Since the fringe pattern and the fringe dimensions are solely a function of the angle between the two interfering beams, the feature size may be made as small as desired, limited only by the processing parameters.
When used in the production of field emission devices, laser interference lithography may require laser beams to be expanded to very large dimensions, e.g. as large as 1 meter in diameter. The intensity profile of the light across the large area laser beam should be as uniform as possible (within .+-.10%). Uniform laser intensity is desired in order for the photoresist to be patterned within the required size tolerances.
An accurate beam profile may be used to monitor and adjust the uniformity of the beam. Furthermore, because the optical components, such as mirrors, may be very large for such large diameter laser beams, it can be difficult to align the laser beam to the center of these optical components without an accurate intensity profile of the beam. It is therefore desirable to be able to accurately monitor the laser beam intensity profile and have the capability to adjust the beam's intensity and direction if necessary.
Current methods of measuring laser beam intensity cannot accommodate the large beam sizes and accuracy requirements of laser beams used with laser interference lithography. For example, U.S. Pat. No. 4,828,384 issued to Plankenhom et al., discloses a high power laser beam intensity mapping apparatus. The Plankenhom system is designed to obtain profiles of high power industrial use lasers on the order of 15 kW or more. The lasers disclosed in the Plankenhom system are larger and more powerful than the lasers used for laser interference lithography, which are typically on the order of 1 W and have a smaller diameter than the high power lasers. The system disclosed in the Plankenhorn patent, does not analyze the entire beam cross-section (i.e. a diameter),but instead only samples a slit-shaped segment of the beam. However, due to variances between the segment and the remaining portion of the beam this method will not always provide an accurate indication of the intensity beam profile.
The current methods of profiling laser beams do not provide the accuracy required for laser interference lithography. Therefore, there is a need for a method and apparatus for more accurately profiling a large area laser beam for use with laser interference lithography.